1. Field of the Invention
This invention relates to a rear-projection screen used in transmission-type projection television systems.
2. Description of the Related Art
Transmission-type projection television systems (hereinafter "rear-projection TVs") are image display systems in which an optical image is enlarged and projected onto a rear-projection screen from a CRT (cathode-ray tube), a liquid crystal panel or the like through a projection lens so that a large-screen picture can be obtained.
FIG. 7 shows an example of the constitution of such a rear-projection TV. As shown in the drawing, in the rear-projection TV, optical images sent from CRTs 1 respectively corresponding R (red), G (green) and B (blue) are enlarged through projection lenses 2 and formed as an image onto the surface of a rear-projection screen 3. The rear-projection screen 3 is comprised of a set of two sheets one of which is a Fresnel lens 4 for directing the light made incident from the projection lenses, toward the position of a viewer and the other of which is a light-diffusing member such as a lenticular lens sheet for dispersing the light emerging from the Fresnel lens 3, in the horizontal and vertical directions to expand a visual angle.
This Fresnel lens 4 is formed of a convex lens, having the structure wherein the lens surface of a convex lens shown in FIG. 8 is divided in a concentric form and divided surfaces are stepwise arranged over a plane as shown in FIG. 8. Thus, the Fresnel lens 4 has on its lens surface a Fresnel surface 4x serving as a lens surface of the convex lens and a surface called a rise surface 4y formed between the divided Fresnel surfaces. As also shown in FIG. 9, an angle formed by the plane surface of the Fresnel lens 4 and the Fresnel surface 4x is called a Fresnel angle .eta., and an angle formed by a normal of the Fresnel lens 4 and the rise surface 4y is called a rise angle .theta..
Fresnel lenses are usually produced by the 2P (photopolymerization) process or compression molding, where a mold (a tooling) obtained by cutting a metal plate or the like with a lathe is used. The cutting to obtain the mold is carried out using a cutting tool having a nose angle of about 30.degree. to about 90.degree.. In this cutting, as shown in FIG. 12, a side y of a mold 7, the side shaped after the nose of a cutting tool has run, forms the rise surface of the Fresnel lens, and a cut surface x, the surface shaped in the direction the cutting tool 6 drives, forms the lens surface of the Fresnel lens. So long as the Fresnel angle .eta. is small enough for the sum of the Fresnel angle .eta. and the nose angle of the cutting tool to be 90.degree. or less, the Fresnel angle .eta. is determined by the position of the cutting tool 6 (the inclination of its surface with respect to the plane of the Fresnel lens), and the rise angle .theta. by the direction in which the cutting tool 6 drives. Thus, the Fresnel angle .eta. and the rise angle .theta. are determined independently of each other. On the other hand, as the Fresnel angle .eta. becomes larger, the mold 7 comes to be cut with the cutting tool 6 on the both sides of its nose, and also the rise angle .theta. becomes larger with an increase in the angle .eta.. For example, assume that a cutting tool having a nose angle of 50.degree. is used, the rise angle .theta. is as follows when the Fresnel angle .eta. is larger than 40.degree.: EQU Rise angle .theta..gtoreq.(Fresnel angle.eta.+nose angle 50.degree.)-90.degree.
FIG. 1 shows the relationship between radii of Fresnel lenses and rise angles thereof. As shown therein by Case-(a), a conventional Fresnel lens has a region having a constant rise angle .theta. determined by a cutting angle of the nose and an outlying region in which the rise angle .theta. has become larger with an increase in the Fresnel angle .eta.. The rise angle .theta. is kept at about 1.degree. in the region in which the rise angle .theta. is constant in Case-(a). This is to make it easy to release a molded product from the mold.
Incidentally, in recent years, the rear-projection TV as shown in FIG. 7 is required to have a smaller depth, and the distance between the projection lens 2 and the rear-projection screen 5 has become shorter accordingly. In such a case, the size of each CRT 1 and each projection lens 2 can not be changed because the brightness of the picture must be ensured. Hence, the optical axes of light rays from the red R and blue B projection systems with respect to the optical axis of light ray from the green G projection system each have a larger angle (hereinafter "convergent angle") .epsilon.. This has caused the problem of a lowering of white uniformity, which is a difference in color tones at some positions on the screen being viewed. For example, in the case of a TV set having a diagonal of 40 inches and a large convergent angle, a problem may occur such that, as shown in FIG. 10, yellow and cyan coloring is seen in an intermediate region extending over a radius of about 200 to 300 mm around the center of its screen when white-raster signals are inputted and the screen is viewed at a distance of about 3 m in front of the rear-projection screen.
Another problem may also occur such that, as shown in FIG. 11, coloring called color corn is seen in a central region standing at about 150 mm above the center of the screen when white-raster signals are inputted to a rear-projection TV and the screen is viewed at a distance of about 1.5 m in front of the rear-projection screen and at an angle of 30.degree. upwards.